Hillary Clinton on Tuesday told ABC News’ David Muir that using a personal email account while Secretary of State was a “mistake” and that she is “sorry” for it.

“That was a mistake. I’m sorry about that. I take responsibility,” the Democratic presidential candidate told Muir in an exclusive interview in New York City.

This is the farthest Clinton has gone yet in offering an apology for her use of a private email server while Secretary of State.

Clinton told the Associated Press on Monday that she would not apologize for it because “what I did was allowed.”

While Secretary of State, Clinton used a private email server based out of her home in Chappaqua, New York, to send and receive work-related emails. Her decision to do so has raised questions over classified material.

Clinton has since turned over more than 55,000 pages of emails from her email server to the State Department, which are being released in batches. (Clinton’s team deemed roughly 31,000 emails to be “personal and private.” Those were not turned over and have since been deleted.)

Last month, Clinton also turned over her private server and thumb drive to the Justice Department amid a federal investigation into the security of the server and whether there was classified information in the emails from the private account she was using.

On Tuesday, Clinton maintained that she did not send or receive classified material on the account and reiterated she is “trying to be as transparent as I possibly can” in her handling of the email controversy.

Clinton’s interview, her first national television interview with an evening news anchor since launching her campaign five months ago, comes after a rocky summer where she faced growing scrutiny over her email account as well as dropping poll numbers.

A recent ABC News/Washington Post poll found that 53 percent of Americans now see Clinton unfavorably, up 8 points since mid-summer.

During the interview with Muir, Clinton also answered questions about her dropping favorability, Vice President Joe Biden’s possible presidential aspirations, Donald Trump, and Iran.

More of Clinton’s interview with Muir will air tonight on “World News Tonight” at 6:30pm ET and “Nightline” at 12:30am ET.